Plated refractory metals



United States Patent PLATED REFRACTORY METALS Sidney S. Brenner, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York No Drawing. Application May 28, 1954, Serial No. 433,292

19 Claims. (Cl. 204-37) This invention relates to the electrochemical deposition of an adherent, metallic coating on a refractory metal base. More particularly, it relates to the formation of an adherent, stable, continuous coating of a metal, such as, for example, nickel, cobalt or iron and alloys thereof upon an article or object made of a refractory metal, such as, for example, molybdemun or tungsten, or a refractory metal alloy.

As is well known, many metals and alloys commonly used for structural purposes suffer a substantial loss in their mechanical properties when their temperature is elevated. Some metals, however, exhibit much greater resistance to physical changes due to temperature rise and, to some degree, resist corrosion at elevated temperatures. These metals are commonly referred to as refractory metals. One such metal is molybdenum. One difliculty encountered in the use of molybdenum and molybdenumrich alloys at high temperatures is the mode of corrosion which has, in the past, limited the use of molybdenum and molybdenum-base alloys as a high temperature structural material to either a non-oxidizing environment, or to use at temperatures below about 700 C, in an oxidizing atmosphere. The explanation of this specific thermal frontier is that molybdenum forms a protective oxide film on its surface which effectively prevents progressive oxidation. However, the melting point of this oxide is about 795 C. In the molten state, oxygen freely diffuses through this film, permitting rapid, progressive, drastic corrosion of the metal. Tungsten and tungsten-base alloys behave in a similar manner, but at somewhat higher temperatures.

Many attempts have been made to protect the surface of molybdenum and molybdenum alloy articles and objects to prevent this high temperature corrosion. For example, it has been proposed that a film of a more chemically stable, oxidation-resistant different metal be applied to the molybdenum surface by cathodic plating. One metal which has acceptable physical and chemical'characteristics for such a coating is nickel. Unfortunately, it has heretofore not been possible to electroplate an adherent nickel coating on molybdenum. In fact, one Widely practiced method of producing high purity nickel foil for laboratory use is by the electrodeposition of nickel upon a smooth molybdenum cathode. This deposited film may be easily peeled from the molybdenum cathode as a sheet or foil after it has attained the desired thickness. If nickel is deposited upon a molybdenum object or article by any previously known method, it forms a quasiadherent film. If the coated article is then subjected to a temperature of the order of 200 C. or higher, the nickel will blister and buckle, thereby separating itself from the molybdenum. This physical phenomenon, caused by the difference between the coefiicients of thermal expansion of the two metals and the fact that the plate is not truly adherent to the base, will occur in any atmosphere.

I have discovered that nickel may be electrodeposited upon a molybdenum article to form an adherent, ther- 2,805,192 Patented Sept. 3, 19.57

ice.

mally stable, protective film, providing an underlying layer of copper and zinc is first applied and heat-treated.

Therefore, a principal object of this invention is the provision of a refractory metal article or object having a thermally stable, adherent, protective metallic film coating.

A further object of this invention is the provision of a process whereby a refractory metal will be provided with an adherent, thermally stable, protective metallic film coating.

Other and further objects of this invention will become apparent from the following disclosure.

More specifically, I have discovered that astable, ad-; herent nickel, cobalt or iron plate may be applied to a molybdenum or tungsten base by the following procedures.

The elements iron, cobalt and nickel comprise a somewhat unique sub-group in the Periodic Table or arrange ment of elements and are conventionally and conveniently referred to as the feromagnetic elements. Itis to be understood that hereafter, in this specification and claims, the word ferromagnetic and the term ferro-' magnetic elements or the like are intended to mean this sub-group.

As an example of my invention, I shall now disclose procedures whereby an adherent nickel plate may be i applied to molybdenum or a molybdenum-base alloy.-

The molybdenum or molybdenum alloy object is first cleaned by any suitable manner. any conventional degreasing using carbon tetrachloride or a similar solvent is advisable. After drying, the article is then immersedin a molten salt bath, preferably molten NaNOz, to additionally clean the surface. This step is optional and is not necessary to successfully plate the article.

If the article is made from substantially pure molybdenum, only such treatment is necessary as may be needed to remove salt from the bath which may adhere to its surface.

If however, the article is made from a molybdenum-- rich alloy, a residual surface film other than salt may be present. This may be readily removed, for example, by

electrolytic cleaning in a sulfuric acid bath in which the article is made the cathode and an insoluble anode provided. Since such an electrolytic cleaning process is con plating procedure may be employed, it has been found that an aqueous bath containing about 180 grams per liter CuSO4'5H2O and 45 grams per liter H2804 is satisfactory. The plating is preferably accomplished at a bath temperature of about 68 to 125 F. The time required varies from 10 to 20 seconds, depending upon the current density, which may be between about 40 to amperes per square foot. This will produce a copper strike of proper thickness. from the bath and rinsed.

The copper-plated article is then subjected to a second electroplating operation in which a zinc strike or film of approximately 10- to 10- centimeters thickness is applied over the copper plate. Again, this coating may be applied by any conventional, suitable electroplating protate), and 119'grams per literglucoseis satisfactory. The i It has been found that.

The plated article is then removedplating is preferably accomplished while maintaining the bath at a pH of 3.5 to 4.6 and a temperature about 75 to 120 F. The time required varies from to seconds, depending upon the current density, which maybe between about 20 to amperesper square foot; Afterplat ing, the article is removed from" the bath and rinsed.

The plated article is then subjected to? an anneal in a non-oxidizing atmosphere at about 900 to 1000 C. for ffom about 5 to 15 minutes. This treatment apparently causes an alloying of the copper and zinc, some diffusion of the plated materials into the article, and finally a pro gressive dezincification of the copper-zinc plate by vaporization. I

It should be noted here that a single brass plating step may be substituted for the separate copper and zinc plating' steps with equal success. Any suitable, conventional brass electroplating bath; and'plating procedure may be employed. It has been found that an aqueous bath containing about 30 grams per liter CuCN, 9.5 grams per liter Zn(CN)2,- 53 grams per liter NaCN, and 30 grams per liter NaHCOz' is satisfactory.v The plating is preferably accomplished at a bath temperature. of about to F. The time required varies from 10 to 20 seconds, depending upon the current density which may be about 2 to 5 amperes per square foot. After plating, the article is removed from the bath and rinsed. The brass-plated article is subjected to the same anneal as set forth supra for the copper and zinc plated article.

After the dezincing anneal, the article is then subjected to a conventional nickel plating operation.

An aqueous bath containing about 330 grams per liter NiSO4, 45 grams per liter NiClz and 37 grams per liter boric acid is satisfactory. The plating is preferably accomplished while maintaining the bath at a pH of about to 4.5 and a temperature of about l15 to F. A current density of between 25 and 100 amperes per square foot should be maintained. The plating time may be varied depending upon the desired thickness.

A molybdenum or molybdenum alloy article plated with nickel in accordance with either of the two schedules described above may be subjected to temperatures substantially higher than 200 C. without the plating separating or blistering or buckling. In fact, the plating is physically stable upon the base article and protects it from oxidation evenat temperatures of the order of 1000 C. in an oxidizing atmosphere.

In a similar manner, alloys of nickel may be plated upon a molybdenum or molybdenum alloy article. One such alloy which has been successfully plated isa nickelmanganese, the plated alloy containing about 7 Weight percent manganese, balance nickel. This alloy is deposited upon a molybdenum or molybdenum alloy article which has been treated according to the schedule recited supra, from an aqueous bath containing about to 300 grams per liter MnSOr-SHzO, 40 grams per liter NlSOi7H20 and 75 grams per liter (NI-192804. The bath is maintained at room temperature during plating and a current density of about 26 amperes per square foot is required. The resulting alloy plate is adherent and physically stable.

Adherent, stable coatings of cobalt, iron. and alloys thereof may be plated on molybdenum and molybdenum alloy articles by merely substituting an appropriate bath and final plating schedule for the abovedescribed nickel arid nickel alloy baths and schedules. For example, iron may be deposited upon such an article which has been prepared as described supra, i. e., having a dezincified copper-zinc base plate, from an aqueous bath containing about 450 grams per liter FeCl2-4H 2O and 150 grams per liter CaClz. The temperature of the bath' is maintained 17 grams per liter NaCl and 45 grams per liter boric acid. The plating may be" accomplished at room temperature and a current density of about 35 to amperes per square foot. The bath should be maintained at about pI-IS.

I have also discovered that the previously described plating techniques may be applied equally well to tungsten and tungsten-base alloy articles to produce a protective, stable, adherent coating thereon of the ferromagnetic metals, i. e., iron, nickel and cobalt, and alloys thereof.

I have further discovered that while the zinc coinponent of the intermediate film is to all intents and purposes entirely removed by volatilization during the heat treatment, leaving a residual film or coating of substantially pure copper, an intermediate plate or film of copper may not be successfully substituted for either the brass intermediate or for the composite copper-zinc intermediate. In every case in which such a substitution was attempted, the final ferromagnetic metal plate did not adhere to the base element when heated to a temperature of the order of 200 C. Therefore, even though the zinc is apparently removed except for extremely small amounts by volatilization during the heat treatment, it must be present in substantial amounts prior to heat treatment.

While I have described the mode of corrosion of certain 1 refractory metals at elevated temperatures in an oxidizing atmosphere, and have disclosed how an adherent coating of a ferromagnetic metal may be applied to protect an can be produced by my invention will readily occur to one skilled' in the art to which they pertain.

It should be further appreciated that the specific plating bath solutions and plating schedules which I have recited are only to be construed as exemplary in their details and any particular, equivalent bath and schedule could be substituted to provide an equivalent individual coating,

In view of the foregoing, it will be seen that I provide a superior and unique method for applying an adherent, continuous coating or plate of a ferromagnetic metal or alloy thereof upon a refractory 'metal such as tungsten or molybdenum and alloys thereof, and articles of refractory metal having a unique, adherent, physically stable coating or plate of the ferromagnetic metals and alloys thereof.

I, therefore, intend in the appended claims to cover all changes and modifications of the examples of my in-' vention herein chosen for purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the' United States is:

1. A method of providing an adherent metallic film on an article consisting of a metal selected from the group consisting of tungsten, molybdenum, tungsten base alloys and molybdenum base alloys comprising the steps of coating the surface of the article with a fihn of metallic copper and zinc by electrodeposition, annealing the coated article in a non-oxidizing atmosphere at about 900 to 1000 C. until the zinc has been substantially complete- 1y volatilized and electrodepositing a metallic ferromagnetic film over the dezincified residual copper film.

2. The method recited in claim 1 in which the film of copper and zinc is applied in two steps comprising the electrodeposition of a copper coat from a copper plating bath and subsequently the deposition on the copper coat of a zinc coat electrodeposited from a zinc plating bath.

3. The method recited in claim 1 in which the article is composed of tungsten and the ferromagnetic film comprises an electrodeposited coating of a metal selected from the group consisting of iron, nickel, cobalt, iron base alloys, nickel base alloys and cobalt base alloys.

4. The method recited in claim 3 in which the electrodeposited ferromagnetic film is composed of nickel.

5. The method recited in claim 3 in which the electrodeposited ferromagnetic film is composed of a nickel base alloy.

6. The method recited in claim 3 in which the electrodeposited ferromagnetic film is composed of iron.

7. The method recited in claim 3 in which the electrodeposited ferromagnetic film is composed of an iron base alloy.

8. The method recited in claim 3 in which the electrodeposited ferromagnetic film is composed of cobalt.

9. The method recited in claim 3 in which the electrodeposited ferromagnetic film is composed of a cobalt base alloy.

10. The method recited in claim 1 in which the article is composed of molybdenum and the ferromagnetic film comprises an electrodeposited coating of a metal selected from the group consisting of iron, nickel, cobalt, iron base alloys, nickel base alloys and cobalt base alloys.

11. The method recited in claim in which the electrodeposited ferromagnetic film is composed of nickel.

12. The method recited in claim 10 in which the electrodeposited ferromagnetic film is composed of a nickel base alloy.

13. The method recited in claim 10 in which the electrodeposited ferromagnetic film is composed of iron.

14. The method recited in claim 10 in which the electrodeposited ferromagnetic film is composed of an iron base alloy.

15. The method recited in claim 10 in which the electrodeposited ferromagnetic film is composed of cobalt.

16. The method recited in claim 10 in which the electrodeposited ferromagnetic film is composed of a cobalt base alloy.

17. The method recited in claim 1 in which the film of copper and zinc is applied in a single step comprising the electrodeposition of a brass coat from a brass plating bath.

18. The method recited in claim 11 in which the film of copper and zinc is applied in two steps comprising the electro-deposition of a copper coat from a copper plating bath and the deposition of a zinc coat electrodeposited from a zinc plating bath.

19. The method recited in claim 12 in which the film of copper and zinc is applied in two steps comprising the electro-deposition of a copper coat from a copper plating bath and the deposition of a zinc coat electrodeposited from a zinc plating bath.

References Cited in the file of this patent UNITED STATES PATENTS 1,524,448 Murphy Jan. 27, 1925 1,597,189 Gero Aug. 24, 1926 2,115,749 Rubin May 3, 1938 2,221,562 Wernlund Nov. 12, 1940 2,392,456 Brown et a1. Jan. 8, 1946 2,683,305 Goetzel July 13, 1954 2,697,130 Korbelek Dec. 14, 1954 FOREIGN PATENTS 717,985 Great Britain Nov. 3, 1954 

1. A METHOD OF PROVIDING AN ADHERENT METALLIC FILM ON AN ARTICLE CONSISTING OF A METAL SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN, MOLYBDENUM, TUNGSTEN BASE ALLOYS AND MOLYBDENUM BASE ALLOYS COMPRISING THE STEPS OF COATING THE SURFACE OF THE ARTICLE WITH A FILM OF MATALLIC COPPER AND ZINC BY ELECTRODEPOSITION, ANNEALING THE COATED ARTICLE IN A NON-OXIDIZING ATMOSPHERE AT ABOUT 900* TO 1000*C. UNTIL THE ZINC HAS BEEN SUBSTANTIALLY COMPLETELY VOLATILIZED AND ELECTRODEPOSITING A METALLIC FERROMAGNETIC FILM OVER THE DEZINCIFIED RESIDUAL COPPER FILM. 